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Dewatering aspects at the forming section of the paper machine: Rewetting and forming fabric structure
Karlstad University, Faculty of Health, Science and Technology (starting 2013), Department of Engineering and Chemical Sciences.
2017 (English)Licentiate thesis, comprehensive summary (Other academic)
Abstract [en]

The underlying motives of the research undertaken here are twofold: to obtain a deeper understanding of the dewatering mechanisms at the forming section of a papermaking machine and to develop numerical models that describe the flow through forming fabrics. More comprehensive knowledge of dewatering in the forming section allows suggestions to be made for improvements that reduce the amount of energy used in the process without affecting the quality of the end product.

 

The objective of this thesis is to answer the following questions:

  • How and why does rewetting occur at the high vacuum suction boxes?
  • How does the structure of the forming fabric affect dewatering at the forming section?
  • Is it possible to create accurate numerical models for forming fabrics, and can these be used to predict the dewatering behaviour of new types of fabrics?

 

Laboratory and pilot studies simulating high vacuum suction boxes were performed together with numerical modelling of the flow of air and water through both the forming fabric and the paper sheet.

 

The conclusion drawn from the pilot study is that rewetting significantly lowers the dryness of the paper sheet exiting the suction boxes. The phenomenon is extremely rapid and is most likely driven by capillary forces. The high speed at which this rewetting occurs makes it difficult to impede by placing the suction boxes closer to the couch pick-up: the solution is more likely to be the use of new and improved designs of the forming fabric. The structure of the forming fabric has been shown to affect the dewatering rate at certain conditions of vacuum dewatering, and can possibly be connected partly to the fact that fibres penetrate the surface of the fabric to varying degrees and partly to the flow resistance of the different fabric structures. Numerical models of high accuracy can be constructed and used to predetermine how new fabric designs would affect dewatering at the forming section.

 

This thesis quantifies aspects of dewatering such as rewetting and the influence of the forming fabric. Understanding these dewatering aspects further provides for the potential enhancement of energy efficiency in the forming section, and thereby the entire papermaking process. The forming fabric can play an important role in improving energy efficiency: rewetting after the high vacuum suction boxes occurs more rapidly than was previously known, so its design might be the only possible way of impeding it. The forming fabric can also improve the rate of dewatering: it is therefore likely that its design will be important in the next stage of developing energy efficiency and thereby play a part in achieving a more sustainable future.

Abstract [en]

This thesis quantifies aspects of dewatering such as rewetting and the influence of the forming fabric. Understanding these dewatering aspects further provides for the potential enhancement of energy efficiency in the forming section, and thereby the entire papermaking process. The forming fabric can play an important role in improving energy efficiency: rewetting after the high vacuum suction boxes occurs more rapidly than was previously known, so its design might be the only possible way of impeding it. The forming fabric can also improve the rate of dewatering: it is therefore likely that its design will be important in the next stage of developing energy efficiency and thereby play a part in achieving a more sustainable future.

Place, publisher, year, edition, pages
Karlstad: Karlstads universitet, 2017. , 51 p.
Series
Karlstad University Studies, ISSN 1403-8099 ; 2017:5
Keyword [en]
Rewetting, forming fabric, forming section, dewatering, web dryness, high vacuum suction box, vacuum dewatering, forming fabric design, numerical model
National Category
Chemical Engineering
Research subject
Chemical Engineering
Identifiers
URN: urn:nbn:se:kau:diva-47640ISBN: 978-91-7063-744-5 (print)ISBN: 978-91-7063-745-2 (electronic)OAI: oai:DiVA.org:kau-47640DiVA: diva2:1065407
Presentation
2017-03-08, 9C203, Karlstads Universitet, Universitetsgatan 2, 65188, Karlstad, 13:15 (English)
Opponent
Supervisors
Available from: 2017-02-16 Created: 2017-01-16 Last updated: 2017-02-16Bibliographically approved
List of papers
1. Rewetting after high vacuum suction boxes in a pilot paper machine
Open this publication in new window or tab >>Rewetting after high vacuum suction boxes in a pilot paper machine
2015 (English)In: Nordic Pulp & Paper Research Journal, ISSN 0283-2631, E-ISSN 2000-0669, Vol. 30, no 4, 667-672 p.Article in journal (Refereed) Published
Abstract [en]

Increased energy efficiency is a major concern for all companies today. Not only does the cost efficiency follow energy efficiency but also environmental and sustainability aspects motivate more energy efficient production lines. A study has been made on a pilot paper machine with the purpose to show the magnitude and time of rewetting after high vacuum suction box dewatering. The grammages used in this study were 20 and 100 g/m2 to cover both tissue and printing paper grades. Machine speed was varied from 400 to 1600 m/min and the maximum pressure drop in the suction box was 32 kPa. The pulp used was unbeaten, chemical, fully bleached softwood from Sweden. Rewetting is observed when the dewatering in the suction box is sufficiently high. No rewetting takes place when the dewatering in the suction box is limited due to insufficient pressure drop and dwell time. The time for the rewetting is in the range of 10-50 ms and in this study the maximum rewetting observed is 180 g/m2, or 6.1% decrease in dryness. The mechanisms behind the phenomenon are believed to be capillary forces caused by sufficiently low sheet moisture and expansion of the network. This study shows that rewetting is so fast that it would be difficult to prevent it without changing major machine parameters.

Place, publisher, year, edition, pages
AB SVENSK PAPPERSTIDNING, Sweden, 2015
Keyword
Energy efficiency, Forming fabric, High vacuum, Paper forming, Pilot study, Rewetting, Suction box dewatering
National Category
Paper, Pulp and Fiber Technology
Research subject
Chemical Engineering
Identifiers
urn:nbn:se:kau:diva-46408 (URN)10.3183/NPPRJ-2015-30-04-p667-672 (DOI)000366040300015 ()
Available from: 2016-10-06 Created: 2016-10-06 Last updated: 2017-01-16Bibliographically approved
2. Influence on sheet dewatering by structural differences in forming fabrics
Open this publication in new window or tab >>Influence on sheet dewatering by structural differences in forming fabrics
2016 (English)In: Paper Conference and Trade Show (PaperCon 2016): Proceedings of a meeting held 15-18 May 2016, Cincinnati, Ohio, USA., TAPPI Press, 2016, 767-776 p.Conference paper (Refereed)
Abstract [en]

Forming fabrics for paper manufacturing are designed with great care to enhance both process and products and are accountable for a lot of the performance of paper machines in the forming section, both with regards to energy and quality aspects. Different approaches to the design of the weave pattern and the choice of yarn materials and diameters have given the market different fabric structures. Fabric parameters that have been shown to cause differences in dewatering are caliper, void volume and permeability. To understand how the structure of the forming fabrics affects sheet dewatering selected fabrics have been tested experimentally, with dewatering equipment that simulates vacuum dewatering.

Dryness of the paper sheet was determined after dewatering and the air volume sucked through sheet and fabric was calculated. The fabrics that were chosen had similar values for all the known parameters previously shown to affect dewatering but had different structures that are defined by the open area (%) in the paper side and the wear side. Tests were performed with three fabric structures and 80 g/m2 softwood sheets. The sheets were made of both unbeaten and highly beaten pulp, and two vacuum levels were used during trials.

The results show that the fabric structure influences the sheet dewatering rate even if the caliper, void volume and permeability are the same. The air volume sucked through the structure of sheet and wire during the dewatering increased linearly with dwell time indicating that a constant air volume was reached. No significant differences were observed between the different fabrics in terms of the air volume at steady state. The conclusions are that the structure of forming fabrics affects the dewatering rate at certain conditions even with constant air volume and outgoing dryness. This is believed to be connected to (i) the fibers’ penetration of the fabric’s surface during the dewatering process or to (ii) the different resistances to in-plane and thickness- direction flow of the fabrics or to a combination of (i) and (ii). Studies of surface topography are used to explain the phenomenon and numerical simulations will be made in a later study to further evaluate this. 

Place, publisher, year, edition, pages
TAPPI Press, 2016
National Category
Chemical Engineering
Research subject
Chemical Engineering
Identifiers
urn:nbn:se:kau:diva-47597 (URN)9781510831193 (ISBN)
Conference
Paper Conference and Trade Show (PaperCon 2016), 15-18 May 2016, Cincinnati, Ohio, USA.
Available from: 2017-01-09 Created: 2017-01-09 Last updated: 2017-01-16Bibliographically approved
3. Modeling of forming fabric structure influence on vacuum box dewatering
Open this publication in new window or tab >>Modeling of forming fabric structure influence on vacuum box dewatering
(English)Manuscript (preprint) (Other academic)
National Category
Chemical Engineering
Research subject
Chemical Engineering
Identifiers
urn:nbn:se:kau:diva-47598 (URN)
Available from: 2017-01-09 Created: 2017-01-09 Last updated: 2017-01-16

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